It is known to employ a metal containing antiwear agent such as zinc dialkyl dithiophosphate in a lubricating composition suitable for a hydraulic system, circulating oil or another industrial oil. However zinc dialkyl dithiophosphate forms deposits such as resins, sludges and varnish in the hydraulic system. These deposits impair the performance of the hydraulic systems by causing valve sticking and unresponsive control. Additionally, the cleaning of these systems is difficult since the deposits are difficult to remove without mechanical abrasion.
International publication WO96/035765 discloses a lubricating composition with a rust reducing or preventing amount of 0.1 to 3 wt % of a metal synthetic aryl sulphonate and 0.01 to 2 wt % of an aliphatic succinic acid or anhydride. Further the use of substituted polyisobutylene succinic acid or anhydride derivatives of polyol esters or polyamines are excluded because lubricating compositions that contain said additive do not exhibit acceptable rust resistance properties.
U.S. Pat. Nos. 4,419,251 and 4,419,252 disclose aqueous lubricants with oil-in-water characteristics containing a dispersant/emulsifier system and an antiwear/rust inhibiting package.
U.S. Pat. No. 5,262,073 discloses a lubricating composition containing a zinc dispersant, 0.3 to 1 wt % of calcium nonyl di-naphthalene synthetic sulphonate detergent and 0.09 to 0.85 wt % of calcium alkylphenate.
U.S. Pat. No. 6,677,281 discloses a lubricating composition containing a metal sulphonate, an ashless alkenyl succinimide and a borated polyolefin dispersant.
U.S. Pat. No. 4,466,894 discloses a composition containing metal salts of phosphorus thio-alcohols, a sulphurised phenate, and a benzotriazole.
International publication WO 93/03121 discloses a metal salt of at least one of a sulphonate, a carboxylate and a phenate, in combination with an aliphatic carboxylic acid or anhydride thereof.
It would be desirable for a lubricating composition to provide acceptable wear performance whilst reducing or preventing deposit formation. The present invention provides a lubricating composition with acceptable wear performance whilst reducing or preventing deposit formation.